The present invention relates generally to implantable cardiac therapy devices, and more particularly, to improved methods for sensing arrhythmias in a pacemaker/defibrillator, and a pacemaker/defibrillator configured or programmed to implement the same.
Implantable cardioverter defibrillators (ICDs) are sophisticated medical devices which are surgically implanted (abdominally or pectorally) in a patient to monitor the cardiac activity of the patient""s heart, and to deliver electrical stimulation as required to correct cardiac arrhythmias which occur due to disturbances in the normal pattern of electrical conduction within the heart muscle.
Cardiac arrhythmias can generally be thought of as disturbances of the normal rhythm of the heart beat. Cardiac arrhythmias are broadly divided into two major categories, namely, bradyarrhythmia and tachyarrhythmia. Tachyarrhythmia can be broadly defined as an abnormally rapid heart rate (e.g., over 100 beats/minute, at rest), and bradyarrhythmia can be broadly defined as an abnormally slow heart rate (e.g., less than 50 beats/minute).
Tachyarrhythmias are further subdivided into two major sub-categories, namely, tachycardia and fibrillation. Tachycardia is a condition in which the electrical activity and rhythms of the heart are rapid, but organized. Fibrillation is a condition in which the electrical activity and rhythm of the heart are rapid, chaotic, and disorganized. Tachycardia and fibrillation are further classified according to their location within the heart, namely, either atrial or ventricular.
A depolarization signal, which is a small electrical impulse, triggers contraction of the myocardial tissue of the human heart. In this regard, the beating of a human heart is manifested by depolarization signals corresponding to the contraction of the atria, referred to as P-waves, and to the contraction of the ventricles, referred to as R-waves. The complex of depolarization signals produced by a normal heart beat is commonly referred to as the PQRS or QRS complex. The sequence of PQRS complexes produced by a beating heart constitutes an electrogram or electrocardiogram signal (depending upon whether the signal is detected within or outside of the heart, respectively) that can be monitored by appropriate electrical circuitry to determine the condition of the heart.
In general, an implantable pacemaker includes sensing circuitry which monitors the heart by analyzing electrograms (EGMs) detected by endocardial (intracardiac) sensing electrodes positioned in or adjacent to the patient""s heart, and pacing circuitry that delivers anti-bradycardia pacing pulses to the heart upon detection of bradycardia, in order to thereby stimulate or pace the heart back into a normal sinus rhythm. More particularly, if the heart does not beat naturally (on its own) within a prescribed time period, (i.e., if an intrinsic heart beat is not detected within a prescribed time period), then an electrical stimulation pulse (pacing pulse) is provided to force the heart muscle tissue to contract, thereby assuring that a prescribed minimum heart rate is maintained. Dual-chamber pacemakers are capable of detecting either atrial or ventricular bradycardia, and delivering the appropriate atrial and/or ventricular anti-bradycardiapacing pulses as required.
In general, an ICD continuously monitors the heart activity of the patient in whom the device is implanted by analyzing electrograms (EGMs) detected by endocardial (intracardiac) sensing electrodes positioned in the right ventricular apex and/or right atrium of the patient""s heart. Contemporary ICDs are generally capable of diagnosing the various types of cardiac arrhythmias discussed above, and then delivering the appropriate electrical stimulation/therapy to the patient""s heart to thereby correct or terminate the diagnosed arrhythmia. As used herein, the terminology xe2x80x9cimplantable cardioverter defibrillatorxe2x80x9d (ICD) is intended to encompass these and other forms and types of implantable cardiac therapy devices.
It is common in implantable cardiac stimulation devices such as pacemakers and ICDs to employ xe2x80x9crefractory periodsxe2x80x9d during which the sensing circuits of the device are inhibited in order to prevent false detection of a cardiac depolarization. More particularly, refractory periods are necessary in such implantable cardiac stimulation devices in order to prevent xe2x80x9coversensingxe2x80x9d. Oversensing is a phenomenon in which a normal cardiac event associated with a depolarization, such as the repolarization of cardiac tissue, referred to as the T-wave, or an after potential generated by a paced depolarization, is sensed and incorrectly determined to be a separate and natural depolarization. Thus, the xe2x80x9crefractory periodxe2x80x9d is defined as the period of time immediately following a natural or induced depolarization during which sensing is inhibited in order to prevent oversensing.
U.S. Pat. No. 3,648,707, issued to Greatbatch, discloses a dual-chamber rate responsive pacemaker which is adapted to operate in an atrial synchronous mode. This type of pacemaker is generally referred to in the art as a xe2x80x9cVDDxe2x80x9d pacemaker. The Greatbatch pacemaker includes electrodes for sensing contractions of the atrium and ventricle, and a pulse generator for pacing the ventricle. After sensing a contraction of the ventricle or pacing the ventricle, a lower rate timer is restarted. If this timer expires, it triggers generation of a ventricular pacing pulse. The Greatbatch pacemaker also includes an A-V interval timer, initiated in response to the sensing of an atrial contraction. On expiration of the timer, the ventricular pacing pulse is triggered. The Greatbatch pacemaker also includes a third timer, defining an upper rate interval initiated following ventricular pacing or sensing of a ventricular contraction. During the upper rate interval, time out of the A-V interval will not trigger a ventricular pacing pulse. This allows for inhibition of the ventricular pulse generator in the event that a natural atrial contraction follows a ventricular contraction. The Greatbatch pacemaker also uses a maximum synchronous pacing rate corresponding to the upper rate interval. If the atrial rate exceeds this rate, the pacing rate is lowered to the higher of one-half of the sensed atrial rate or the rate determined by the lower rate timer. In this way, pacemaker induced or mediated tachycardias (PMTs) are prevented.
U.S. Pat. No. 4,059,116, issued to Adams, discloses a VDD dual-chamber rate responsive pacemaker which, rather than preventing generation of a ventricular stimulus in response to time out of the A-V interval during the upper rate interval, instead delays the ventricular stimulus until the expiration of the upper rate interval. In addition, the Adams pacemaker utilizes a post-ventricular atrial refractory period (PVARP) after each ventricular pacing pulse and each sensed ventricular contraction, during which an atrial contraction does not initiate timing of the A-V interval. Because of these features, the Adams pacemaker exhibits an improved response to atrial contractions occurring at intervals less than the upper rate interval. The Adams pacemaker was programmed to generate ventricular stimulation pulses separated by the upper rate interval, displaying gradually lengthening A-V intervals until an atrial contraction fell within the postventricular atrial refractory period. The Adams pacemaker would then resynchronize on the next subsequent atrial contraction, mimicking the natural condition known as Wenckebach behavior. In commercially marketed pacemakers employing the Adams invention, the behavior of the pacemaker in the presence of high natural atrial rates is referred to as xe2x80x9cPseudo-Wenckebachxe2x80x9d upper rate behavior.
Numerous other dual-chamber pacemakers have been proposed which vary their post-ventricular atrial refractory periods (PVARPs) in an attempt to prevent PMTs. For example, U.S. Pat. No. 4,920,965, discloses a dual-chamber pacemaker in which a post-ventricular atrial refractory interval is calculated based upon the time of occurrence of the atrial contraction, relative to the preceding ventricular pacing pulse or sensed ventricular contraction. The post-ventricular atrial refractory period is gradually decreased in response to increasing natural atrial rates. Alternatively, it has been suggested to monitor the timing of atrial contractions with respect to previous ventricular contractions and if their timing indicates that the atrial contractions are probably retrograde P-waves, to extend the post-ventricular atrial refractory period beyond the measured time of occurrence of the retrograde P-waves. Such pacemakers are disclosed in U.S. Pat. No. 4,544,921 and 4,503,857, both issued to Boute et al.
U.S. Pat. No. 5,129,393, issued to Brumwell, discloses a VDD dual-chamber rate responsive pacemaker equipped with an integral activity sensor for monitoring the physical activity level of the heart patient. The pacing rate of the Brunwell pacemaker is regulated in response to the patient""s need for cardiac output, in response to the output of the integral activity sensor, and is adapted to operate in an atrial synchronous mode. The post-ventricular atrial refractory period (PVARP) is calculated in response to both the sensor-determined ventricular pacing rate and the natural atrial rhythm. The ventricular pacing rate will thus follow a high natural atrial rate, even in the presence of an indication by the sensor of low physical activity. In particular, the PVARP is calculated by determining the average interval separating natural atrial contractions (average A-A interval) and the average interval separating paced or sensed ventricular contractions (average V-V interval). An interval equal to a predetermined portion (e.g., 75%) of the A-A interval (A-A ARP) and an interval corresponding to a predetermined portion (e.g., 75%) of the V-V interval (V-V ARP) are calculated. The V-V ARP and A-A ARP are compared, and the lesser of the two intervals are employed as a variable PVARP.
In addition, an article entitled xe2x80x98The xe2x80x9cAutomatic Mode Switchxe2x80x9d Function in Successive Generations of Minute Ventilation Sensing Dual Chamber Rate Responsive Pacemakersxe2x80x99 (F. Provenier et al., PACE, Vol. 17, pps. 1913-19 (November, 1994)) discloses automatic switching from DDDR to VVIR pacing modes when a selected number of atrial events are sensed by automatic mode switching (AMS) circuitry. According to the article, a PVARP adapts continuously to changes in metabolic indicated rate (MIR) determined by a sensor such as a ventilation sensor. It will be appreciated that the adaptive PVARP includes a blanking period of a predetermined length, e.g., 100 milliseconds. The pacemaker thus is configured so as to permit sensing during the later stages of the PVARP. A refractory period as discussed in the article is defined as consisting of an absolute refractory period (the blanking period) and a relative refractory period (that portion in which sensing of atrial events can occur).
Although the above-discussed dual-chamber pacemakers are generally effective at preventing or terminating PMTs, and/or at adaptively varying the PVARP to allow sensing of fast native atrial rates, while ensuring adequate atrial and ventricular anti-bradycardia pacing, because they do not deliver either atrial or ventricular defibrillation therapy, they are not concerned with the under detection of either atrial or ventricular tachyarrhythmias.
Single-chamber ICDs are designed to detect and treat only ventricular arrhythmias, and not to detect and treat atrial arrhythmias. If a given patient has a cardiac condition which requires both atrial anti-bradycardia pacing and treatment of ventricular arrhythmias, it is possible that both an implantable pacemaker and an ICD would be separately implanted. Obviously, the cost of these separate devices and the cost and risk of the required separate implantation procedures is much greater than would be the cost and risk associated with the implantation of a single device which could perform the functions of both the pacemaker and the ICD. In addition, some ICDs do not allow the pacing rate to vary according to a patient""s hemodynamic requirements. This forces pacing rates in these ICDs to remain fairly slow, e.g. 30-70 pulses per minute. Currently available pacemakers can pace at higher rates, e.g. 150 pulses per minute, when the patient requires it. This is desirable for patient health and well-being. For these reasons, one of the major areas of RandD within the field of ICDs is the development of dual-chamber ICDs which are capable of detecting and treating both atrial and ventricular arrhythmias.
In general, it will be appreciated that a dual-chamber ICD has two primary functions. The first primary function is to provide both atrial and ventricular anti-bradycardia pacing, as required, in order to ensure that an appropriate heart rate is maintained. The second primary function is to detect atrial and ventricular tachyarrhythmias and deliver the appropriate cardiac therapy, as required. However, the first and second primary functions conflict when moderate to high rates of pacing are required, since moderate to high pacing rates require that the device be refractory for a significant portion of the time during the periods when such moderate to high pacing rates are required. This refractoriness can result in undersensing or delayed detection of atrial and/or ventricular tachyarrhythmias. For the sake of simplicity, the ensuing discussion will be directed to the issue of undersensing or delayed detection of ventricular tachyarrhythmias. However, it should be appreciated that the ensuing discussion and the present invention are equally applicable to the issue of undersening or delayed detection of atrial tachyarrhythmias, or most broadly, to any single-chamber of dual-chamber pacemaker/defibrillator in which undersensing or delayed detection or any type of arrhythmia that may be hidden by pacing is an issue.
Two factors can bear on a solution to this problem of conflicting goals in an ICD capable of high pacing rates. The first factor is whether the pacemaker (or xe2x80x9cpacerxe2x80x9d) section and defibrillator (or xe2x80x9cdefibxe2x80x9d) section share the same circuitry for sensing ventricular cardiac events. The second factor is whether the pacer and defib sections share the same ventricular sense refractory periods. These two factors create four possible machine types which may use different methods to solve this problem.
Four different types of dual-chamber ICD are proposed. In a first type of dual-chamber ICD, the pacemaker and defibrillator sections share the same circuitry for sensing ventricular cardiac events and use the same ventricular sense refractory periods. In a second type of dual-chamber ICD, the pacemaker and defibrillator sections utilize completely independent circuitry for sensing ventricular cardiac events and use completely independent ventricular sense refractory periods. In a third type of dual-chamber ICD, the pacemaker and defibrillator sections share the same circuitry for sensing ventricular cardiac events, but use different ventricular sense refractory periods. In a fourth type of dual-chamber ICD, the pacemaker and defibrillator sections utilize completely independent circuitry for sensing ventricular cardiac events but use the same ventricular sense refractory periods. The essence of sharing the sensing circuitry between the pacemaker and defibrillator sections is that the threshold used for sensing intrinsic cardiac events is always the same for both pacemaker and defibrillator sections, i.e. they see the same cardiac events when neither are refractory.
An exemplary dual-chamber ICD of the first type is disclosed in U.S. Pat. No. 5,007,422, issued to Pless et al., which patent is commonly assigned to the assignee of the present invention and which patent is herein incorporated by reference. It will be readily appreciated by those skilled in the pertinent art that the dual-chamber ICD disclosed in the Pless et al. patent can be easily programmed to function as a dual-chamber ICD of the second type. Different methods of operating the dual-chamber ICD disclosed in the Pless et al. patent are disclosed in U.S. Pat. Nos. 5,111,816 and 5,048,521, which patents are commonly assigned to the assignee of the present invention and which patents are also incorporated herein by reference. However when the dual-chamber ICD disclosed in the Pless et al. patent is programmed to function as a device of the first type or programmed to function as a device of the third type, it is possible that ventricular arrhythmias would be masked or concealed due to the occurrence of such ventricular arrhythmias during the pace refractory period, and not during the xe2x80x9calert periodxe2x80x9d (i.e., non-refractory period) when the sense circuitry is active (i.e., not inhibited).
An exemplary dual-chamber ICD of the second type is disclosed in U.S. Pat. No. 5,470,342, issued to Mann et al., which patent is commonly assigned to the assignee of the present invention and which patent is herein incorporated by reference. More particularly, the Mann et al. dual-chamber ICD utilizes two parallel signal processing channels for sensing depolarization signals sensed over a single sensing lead (or lead network), each processing channel having its own independently programmable refractory period, and each signal processing channel further having its own independently adjustable gain and/or threshold setting. It will be readily appreciated by those skilled in the pertinent art that the dual-chamber ICD disclosed in the Mann et al. patent can be easily programmed to function as a dual-chamber ICD of the fourth type. The Mann et al. dual-chamber ICD is programmed to adaptively adjust the respective refractory periods, and/or gain/threshold settings of the respective processing channels in such a manner as to optimally sense: (1) cardiac depolarizations, whether associated with natural cardiac rhythm or tachyarrhythmias; or, (2) fibrillation. However, the Mann et al. dual-chamber ICD does not fully resolve the problem described above, i.e., it is still possible that ventricular arrhythmias would be masked or concealed due to non-occurrence of a single ventricular arrhythmic event within the alert period during moderate to high rates of pacing.
Therefore, based on the above and foregoing, it can be appreciated that there presently exists a need in the art for improved methods for sensing arrhythmias in a pacemaker/defibrillator which overcome the above-described drawbacks and shortcomings of the existing methods, and a pacemaker/defibrillator programmed to implement the same.
The present invention encompasses, in one of its embodiments, a method of detecting an arrhythmia hidden by pacing in a pacemaker/defibrillator, including the steps of:
(a) increasing a pace cycle length and delaying a next occurring pacing pulse by the increased pace cycle length;
(b) analyzing a sensed cardiac signal to determine the presence of an arrhythmia;
(c) repeating steps (a) and (b) a plurality of times;
(d) decreasing the pace cycle length and advancing the next occurring pacing pulse by the decreased pace cycle length;
(e) analyzing the sensed cardiac signal to determine the presence of an arrhythmia;
(f) repeating steps (d) and (e) a plurality of times;
wherein steps (a)-(f) are performed in a manner so that an average pace cycle length achieved over a period spanning steps (a) through (f) is equal to a desired pace cycle length.
The present invention encompasses, in another of its embodiments, a method of detecting an arrhythmia hidden by pacing in a pacemaker/defibrillator, including the steps of:
(a) increasing a pace cycle length by an amount x;
(b) analyzing a sensed cardiac signal to determine the presence of an arrhythmia;
(c) repeating steps (a) and (b) a plurality V of times;
(d) decreasing the pace cycle length by an amount y;
(e) analyzing the sensed cardiac signal to determine the presence of an arrhythmia;
(f) repeating steps (d) and (e) a plurality Z of times;
wherein steps (a)-(f) are performed in a manner so that an average pace length achieved over a period spanning steps (a) through (f) is equal to a desired pace cycle length.
The method, according to this embodiment, preferably further includes the step of setting a pacer sense refractory period to a value n greater than one half of the pace cycle length, once a hidden arrhythmia has been unmasked. This step can be implemented by setting the pacer sense refractory period to a value that does not exceed one half of the minimum pacing cycle length (CL), or by adaptively varying the pacer sense refractory period in response to variations in cardiac rate in such a manner as to ensure that the pacer sense refractory period does not exceed one half of the pace cycle length.
The present invention, in still another of its embodiments, encompasses a method of detecting an arrhythmia hidden by pacing in a pacemaker/defibrillator, including the steps of:
periodically checking cardiac activity during a relative refractory portion of a pace refractory period for a cardiac signal indicative of a cardiac event; and,
if a cardiac signal is detected during the periodically checking step, then delaying a next occurring pacing pulse by an extension period.
The method, according to this embodiment, preferably further includes the step of additionally checking cardiac activity during the extension period for a further cardiac signal indicative of a cardiac event. If a cardiac signal is detected during the periodically checking step, and a further cardiac signal is detected during the extension period, then the cardiac event is determined to be a hidden arrhythmia. If a cardiac signal is not detected during the periodically checking step, then the next occurring pacing pulse is delivered without any delay.
The present invention, in yet another of its embodiments, encompasses a method of detecting an arrhythmia hidden by pacing in a pacemaker/defibrillator, including the steps of:
periodically checking cardiac activity during a relative refractory portion of a pace refractory period for a cardiac signal indicative of a potential arrhythmic cardiac event;
if a cardiac signal is detected during the periodically checking step, then checking cardiac activity during the relative refractory portion each of a plurality of consecutive pace refractory periods for a further cardiac signal indicative of a potential arrhythmic cardiac event; and,
if a further cardiac signal is detected during each of the plurality of consecutive pace refractory periods, then delaying a next occurring pacing pulse by an extension period.
The method, according to this embodiment, preferably further includes the step of additionally checking cardiac activity during the extension period for an additional cardiac signal indicative of a potential arrhythmic cardiac event. If a cardiac signal is detected during the extension period, then the cardiac event is determined to be a hidden arrhythmia. Alternatively, the method, according to this embodiment, further includes the step of additionally checking cardiac activity during each of a plurality of consecutive extension periods for an additional cardiac signal indicative of a potential arrhythmic cardiac event, and if an additional cardiac signal is detected during each of the plurality of consecutive extension periods, then the cardiac event is determined to be a hidden arrhythmia.
The relative refractory portion of the pace refractory period and/or the extension period are preferably adaptively varied in such a manner as to ensure detection of hidden arrhythmias despite variations in cardiac rate and/or pacing rate.